Vulnerability · Published Jun 23, 2026 · Assigned by GitHub_M

CVE-2026-55517Deno: Denial of service via non-ASCII bytes in WebSocket response headers

Medium 4.3

In plain English

A medium-severity security flaw in deno from denoland. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 25% of all known vulnerabilities).

Severity (CVSS 3.1)4.3Medium

Exploit odds (30 days)0.3%top 75.4% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

Deno is a JavaScript, TypeScript, and WebAssembly runtime. Prior to 2.7.5, a Deno program that opens a client WebSocket connection could be crashed by the remote server. While handling the WebSocket handshake response, Deno parsed the Sec-WebSocket-Protocol and Sec-WebSocket-Extensions response headers in a way that assumed their bytes were always printable ASCII. A response header containing non-visible-ASCII bytes (0x80-0xFF) caused a panic that aborted the entire Deno process. This vulnerability is fixed in 2.7.5.

Affected products (as reported)

Vendor	Product	Versions
denoland	deno	< 2.7.5

Weakness: CWE-248 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L

References

https://github.com/denoland/deno/security/advisories/GHSA-x2qc-cmh9-f4hf x_refsource_CONFIRM